A party wall agreement is a set of terms that both parties agree to as to what color the duplex will be painted, type of landscaping on each side, roof repair if needed and/or no old broken vehicles or campers sitting in the driveway, etc.
You also need to include "shared maintenance issues," such as sewer lines that may belong to one half of the duplex and cross over into the front yard or basement area of the other duplex; as you may need access if there is a problem at some time. The same thing applies to electrical meters. In an older duplex they could be placed on the outside of just one unit and the other party may need access at some point.
These types of problems usually exist with older duplexes before strict codes and guidelines were inforced. The new buyer or the current owner on the other side of the duplex should contact their attorney and have a "party wall" agreement drawn up to the satisfaction of the buyer and neighbor. They could split the cost of it because the purpose is to protect both of them.